Final answer:
Human activity is the primary cause of the greenhouse effect.
Step-by-step explanation:
The primary cause of the greenhouse effect is a) Human activity. Human activities such as burning fossil fuels, agriculture, and land clearing are increasing the amount of greenhouse gases released into the atmosphere. Carbon dioxide (CO₂) is the primary greenhouse gas emitted through human activities, and it accounts for about 82.2% of all U.S. greenhouse gas emissions. These human-related emissions are responsible for the increase in greenhouse gases in the atmosphere since the industrial revolution.